I'm planning to write little posts highlighting the works of a lot of the true masters of horror fiction, including a bit about them and what I consider to be their most enjoyable yarns. One aspect that I will address is the main 'theme' of the author's work, and I wanted to say a few words about what I mean by that.
In all of creative writing, authors tend to have settings, topics, or metaphors that appeal to them and which they return to again and again. The one place I don't usually hear this discussed is in the genre of horror fiction, perhaps because horror is generally considered to be a 'low' form of writing. Such 'themes' of a writer can be very insightful, both in understanding the author himself and the times he lived in as well as in understanding what makes certain authors' fiction effective.
To consider a few illustrative examples, which we'll no doubt return to in detail:
H.P. Lovecraft performed some of his greatest work in the subject of 'cosmic horror', which emphasizes a universe which is indifferent and incomprehensible to the insignificant humans who dwell within it. Stories like The Dunwich Horror and The Colour Out of Space deal with entities that do not satisfy the laws of physics as we know them, while stories like At the Mountains of Madness and The Shadow Out of Time emphasize man's insignificance in the history of the Earth. Lovecraft clearly drew a lot of inspiration from the discoveries in physics and other natural sciences which were beginning to permeate the public's consciousness.
Edgar Allen Poe was fixated on the subjects of death and burial (particularly prematurely), as illustrated in numerous stories such as The Cask of Amontillado, The Fall of the House of Usher, The Masque of the Red Death, and The Premature Burial. Incidentally, I highly recommend reading the latter of those stories, which is more than it appears.
As a little less literary of an example, it is worth noting that director James Cameron had an apparent long-time obsession with nuclear destruction, which appeared as a major plot point in nearly all of his films up until Titanic: The Terminator, Aliens, The Abyss, Terminator 2, True Lies.
It's worth mentioning that a writer's theme is not a straitjacket: writers can and do venture often from them. They do serve, as I said, as an interesting window into the writer's interests, and I'll try and deduce themes of the various masters as I discuss them.
And my own theme as a fiction writer? Looking back on my stories, there is a HUGE emphasis on stories which contrast supernatural monsters with the more mundane real-world variety, and often pit them against one another.